In the production of composite wood panels or boards such as waferboard, plywood, oriented strand board (OSB), particleboard, medium density fibreboard (MDF) and the like, it is desirable to monitor the stiffness and strength of the end product which are typically flat sheets. The stiffness (EI) or the modulus of elasticity (MOE), which can be derived from stiffness, provide an indication of the ultimate strength or modulus of rupture (MOR) of the panel. Since composite wood panels and boards are generally formed on an assembly line, it is also desirable to have testing done at the output end of the assembly line in a testing unit designed for high speed operation.
It is possible to take individual selected panels off the assembly line and subject them to appropriate testing, however, this testing scheme tends to introduce a delay between production and testing and does not lend itself to efficient feedback control for the manufacturing process.
Prior art equipment exists for automatically testing panels or lumber as they exit the production line. This existing equipment generally performs testing by bending of the material. Bending can be done by introducing the panel or lumber into an "S" shaped path. The material is forced to deflect a given amount in its elastic range in two opposite directions and the resultant forces are measured using load cells to determine panel stiffness. The "S" shaped testing method produces an average panel stiffness of both sides of the test panel and assumes a linear stiffness response of the test panel. Examples of prior art testing apparatus and methods that rely on the deflection of the panel or lumber into an "S" shape are U.S. Pat. No. 3,196,672 to Keller and U.S. Pat. No. 4,708,020 to Lau et al.
An alternative to "S" shaped bending of the panel is to subject the panel to bending by applying forces to distort the panel sequentially to two given deflections on the same side of the panel while simply supporting the ends of the panel. This technique can be referred to as "W" shaped bending as the two sequential bending tests, when imagined side by side, distort the panel into a shallow "W" shape. The load to produce each of the given deflections is monitored and the panel stiffness and modulus of elasticity can be determined by the slope of the load-deflection curve. Examples of prior art testing equipment that relies on "W" shaped bending of the panel are U.S. Pat. No. 4,722,223 and U.S. Pat. No. 5,804,738 both to Bach et al. To avoid non-linear regions of the load-deflection curve, two points along the linear region are used to determine the slope rather than relying on one data point and the origin. Preferably, the two data points are determined by applying a first small pre-load of approximately 10% of the ultimate load for the panel, and then applying a final load of approximately 30% of the ultimate load.
Unlike "S" shape bending which measures stiffness based on both sides of the panel, "W" shape bending measures the panel stiffness from one side of the panel corresponding to the intended load bearing side of the panel. Test results indicate that there can be a difference in panel stiffness of up to 6% between opposite sides. Therefore, the "W" shaped bending test is the preferred method for determining the stiffness of load bearing panels.